Chimpanzee skin cultures have been established and distributed; their response to viral and chemical carcinogens has been studied and resulting abnormal, possibly preneoplastic lines identified. Similar human cultures have also been established in preparation for chimpanzee tumor vaccine trials. In vivo behavior of recombinant mouse type C viruses has aided in the identification of specific genes essential for infectivity and malignancy. Studies on the maintenance and behavior analysis of human and other xenotransplants in athymic nude mice are continuing. Studies on photochemical treatment of tumor cells and tumor viruses show promise in selective killing of malignant cells.